This study will record patient electrocardiograms - as well as various other body signals (e.g., blood pressure, pulse wave, peripheral hemoglobin oxygen saturation) - as monitored routinely both during and immediately subsequent to surgical operations performed under general anesthesia. Through retrospective analysis of patients' heart rate variability (HRV), in concert with the other abovementioned parameters, the investigators seek to forward the development of novel mathematical models and tools for on-line detection of cardiovascular and autonomic dysfunction in the perioperative setting.